The use of optical transmission paths is preferred in large telecommunication networks, because of their special quality characteristics, such as e.g. high transmission capacity, low damping and no crosstalk. Several subscriber connections are routed through an optical transmission path (optical waveguide), e.g. by the frequency multiplex process or the time multiplex process.
The article "16 Gbit/s Fibre Transmission Experiment using Optical Time-Division Multiplexing" by R. S. Tucker et al., published in "Electronics Letters", 1987, Vol. 23, No. 24 on pages 1270 and 1271, documents an optical time multiplex system. In it, four parallel electrical data streams are converted by optical senders and Ti:Li Nb 03 modulators into four parallel optical data pulse streams. The optical senders are time-displacement-controlled by electrical time delay elements (quarter bit-period time delays) in such a way, that the parallel optical data pulse streams are converted without overlapping into an optical waveguide in a space multiplex stage (MUX). The recovery of the four electrical data streams at the end of the optical waveguide is done by means of a demultiplexer stage (DEMUX), which contains cascaded optical directional coupler switches. Four optical receivers are installed downstream of the demultiplexer stage.
The introduced optical time multiplex system is self-contained. The conversion of the electrical data streams by means of transmitter-switching and receiver-switching to the optical waveguide takes place at its beginning or its end.
It is not possible to integrate the transmitter-switching and the receiver-switching into any section of a given optical transmission path.
It should also be possible to attach as many data sources and data outputs as possible to an optical transmission path, when they are located individually or in groups in different sections of the transmission path. This problem occurs particularly in ring structure transmission systems or in large scale switching networks, such as ATM systems. The publication "NTT Review", 1991, Vol. 3, No. 2, on pages 98-101 of the article "Photonic Switching Technologies . . . " by K. Yukimatsu et al., introduces a high speed optical ATM network with a transmission speed of about 1 Tbit/s.
In it, the ATM cells of several parallel data inputs are compressed in time and then are interlaced without overlapping and transmitted through an optical time multiplex path.
At the end, the compressed ATM cells are assigned to parallel data outputs by means of cell selectors, and are expanded in time to their original form by means of cell buffers. The article does not provide sufficiently clear technology, particularly in the depiction of the "cell" elements, in which inconsistencies between text and drawing occur. Still, it can be seen in principle that the transmission of data takes place in cells in the time multiplex process, and that neither the transmitter-switching on the input side, nor the receiver-switching on the output side, can be integrated into any section of a given optical transmission path.